1. Field of the Invention
The present invention relates to a non-impact recording apparatus and method for recording.
2. Brief Description of the Prior Art
In U.S. Pat. No. 5,255,013, a printhead employing linear arrays of LEDs for recording is described. Light from selective illuminated LEDs impinges upon a photosensitive surface such as a photographic film or electrostatically charged photoconductor to form a latent image. This image may be developed by conventional known means to form a visible image. In forming images that vary in density, it is further known to adjust the size or density of each picture element (pixel) recorded by each LED in steps so that certain pixels may be said to have greater grey levels than other pixels. In recording a grey level pixel, the pixel may be recorded for a single timed duration that varies with a signal representing time duration. See in this regard U.S. Pat. No. 5,586,055, the contents of which are incorporated herein by reference. Another approach disclosed in U.S. Pat. No. 5,255,013 employs a series of rapid flashes to record a single pixel wherein flashes may be either of uniform duration or of varying or binary weighted duration. This can be referred to as multiple address grey level printing since assuming the recording medium is moving relative to the printhead in the in-track direction, the different flashes of light will tend to impinge on slightly different areas of the recording medium; however, the overlap of the various light impingements effectively records a single pixel.
The invention described herein relates to both types of printing.
In order to record a grey level pixel using an LED, multibit exposure data signals need to be generated that are used to control the amount of exposure for each pixel to be recorded by an LED. As noted in U.S. Pat. No. 5,255,013, exposure data signals may be generated by providing a dual look-up table (LUT) set wherein a gray level value data signal is adjusted for nonuniformity of the recording elements and for different process conditions of the recording apparatus. In one of the LUTs an 8-bit word representing uniformity correction or an exposure characteristic of the recording element is provided. Specifically, the uniformity correction LUT represents a grading or binning of the LEDs into say 256 categories according to their respective brightness levels. Thus, where a printhead has a sum of say 7296 LEDs arranged in a row at intervals of 1/600 of an inch, each LED is categorized or associated with a particular bin value from 0-255. Therefore, LEDs which tend to generate about the same light output when driven by the same levels of current are assigned a similar bin value. The second of the dual LUTs is a process LUT which has as its inputs a grey level value data signal, the LEDs bin value and perhaps the color that the recorded image will be developed with. The output of the process LUT is a multibit exposure data signal which may be an 8-bit signal representing exposure value for recording the pixel. The exposure value may be related directly to exposure duration or to a combination of exposure duration and LED intensity. In a printer that is to operate under various very different process conditions it is desirable to provide flexibility to enable accurate rendition of images. The dual LUT approach of the prior art permits some flexibility in that different process conditions due to development with different colors requires very different process conditions. Further, flexibility is also desirable where the gray level signals are defined in different bit depths. For example, the input for one document or image may have a bit depth of 1, i.e. a binary print or no print decision. Other documents or rasterized image data may have respective bit depths of 2, 4 or 8 bits per pixel. Each bit depth needs to be associated with its own process condition in order to provide the best rendition with the given data.
A problem with the dual LUT approach is that for the four color printer case, a process LUT may require four 64K.times.8 SRAMS of memory to store the exposure data. It is therefore an object of the invention to provide a more economical printer apparatus and method that operates in accordance with more efficient use of memory and thereby reduces the need for extra memory required by the prior art.